This invention is directed to an easily interchangeable assembly which carries a solder reflow heater bar so that the heater bar can pivot with respect to the work to apply an even force over several leads being soldered, and is also directed to an associated control system wherein two thermocouples on the heater bar respectively sense the heater bar temperature to control input power and to sense over-temperature to stop the heating cycle when over-temperatures are sensed.
In reflow soldering, a printed circuit board carries pads thereon which are tinned with solder. A component, often with many parallel leads, is put in place with its leads over the pads. A heater bar is brought down into contact and heat is applied. Lack of compliance of the heater bar causes uneven force upon the leads to result in unreliable soldering. Free pivoting of the heater bar does not control the initial angle of the heater bar and this results in lateral motion of the heater bar as it moves down into soldering position. Such lateral motion may swing the leads off of their pads, may cause the leads to contact adjacent pads, or may cause limited lead-path contact with consequent unreliability.
A thermocouple is mounted on the heater bar to sense the heater bar temperature and transmit this sensed data to the power supply. It can be readily determined that the thermocouple has failed by becoming an open circuit, and such detection can control the power supply circuitry to disable the power output when the thermocouple circuit is open. However, present power supply circuitry cannot detect partial or total short circuits in the thermocouple circuit. If this mode of failure occurs in which the thermocouple resistance has deteriorated to a value between nominal resistance and short circuit, with circuit resistance less than nominal, there is a corresponding lower signal amplitude which results in excessive heating, which often causes damage to the printed circuit board and heater bar.
Heater bars are subject to degradation and destruction and occasionally must be changed. Thus, it is also necessary to provide a heater bar assembly which permits quick and easy changing to minimize production down time. Accordingly, there is need for a solder reflow heater bar assembly and control system which is easily changed, which is properly compliant with the level of the leads to be soldered, and is arranged so that thermocouple failure can be sensed before excessive heating of the board takes place.